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Extension Horticulture Team

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Additional Late Blight Outbreaks Continue to be Confirmed in PA

Posted: June 15, 2012

Late blight has now been confirmed in Blair, Franklin, Mifflin, Lancaster and Schuylkill Counties in Pennsylvania.

Foliar late blight lesions on tomato. Photo: Beth K. Gugino

Additional reports of
late blight are continuing to roll in. The latest confirmations since June 13th
include three commercial potato fields in Schuylkill Co. as well as another commercial
field in Mifflin Co. planted with both tomatoes and potatoes. Unconfirmed
reports have also been made on tomato and/or potato in Chester, Huntington and
Indiana Co. If you have not already done so, it is very important that protectant
fungicides should be applied all tomatoes and potato crops in Pennsylvania. If
you are located in a county where late blight has been reported including late
blight specific fungicides in your fungicide program is also highly recommended.
So far there have been no confirmed reports from home gardens although several suspect
samples have been examined.

The first late blight
reports made on June 4th from a commercial tomato field in Blair Co.
and on potato and tomatoes from a commercial farm in Franklin Co. have been
genotyped as US23. This was the most common genotype associated with late
blight outbreaks in 2011. US23 is characterized as an A1 mating type that can infect
both tomatoes and potatoes and is sensitive to mefenoxam (Ridomil). In fields where
this genotype has been identified, mefenoxam will help to effectively manage
late blight. For resistance management, rotate mefenoxam with another a late
blight specific product from another FRAC code tank mixed with a protectant.
These include but are not limited to Previcur Flex (FRAC 28), Ranman (FRAC 21),
Tanos (FRAC 11+27), Curzate (FRAC 27), etc. Keep in mind that the PHIs for
mefenoxam on tomato and potato are 5 and 14 days, respectively.

All confirmed
outbreaks thus far have been in production fields however, keep in mind that
crops grown under high tunnels and other protected structures are not immune
from getting late blight. The pathogen does not necessarily require leaf
wetness that results from precipitation to cause disease. Extended dew periods
that occur with evening cooling and even very high relative humidity can also
create conditions that are favorable to infection and disease development. For
greenhouse tomatoes both Heritage and Catamaran are registered for suppression
of late blight. In fields where late blight has been confirmed, rogueing or
burning down the most severely infected plants or portion of the field will
reduce the build-up of inoculum and the potential for spread within the field,
between fields and between farms. Incorporating the use of late blight specific
fungicides will further reduce the development of new lesions and spread of the
disease. Products like Tanos (famoxadone + cymoxanil) and Curzate (cymoxanil) have
a slight amount of “kick-back” activity and are effective at managing every
early stages in the infection process (all of which are invisible to the naked
eye).

Copper still
remains the most effective tool for organic production. It is important to
apply it preventatively before symptoms are observed and since it is a
protectant, thorough coverage is also very important. Thorough coverage is
important for any type of protectant fungicide. These are only effective where
the active ingredient comes in contact with the plant surface.

Please continue
to scout your fields and communicate with your local extension office or me if
you suspect late blight. I have received numerous phone calls and emails from
people with concerns about late blight and I want to continue to encourage that
level of communication. If you suspect late blight on your farm, please contact
your local county Penn State Extension Office or let me know via email at bkgugino@psu.edu or by phone at 814-865-7328.

Additional images of late blight on tomatoes and
potatoes can be found at the Penn State Extension Vegetable and Small Fruit
website under the Vegetable Disease Images link on the homepage at http://extension.psu.edu/vegetable-fruit. Also for the information regarding where the
latest confirmed outbreaks have been reported and to receive email or text
alerts about when late blight has been confirmed with a personally defined
radius from your location visit http://usablight.org.